Embodiments of the present invention relates generally to the field of multi-phase flow metering and, more particularly, to a method and system for determining salinity in multi-phase fluids.
A multi-phase fluid refers to a composition that includes at least two phases of material. For example, multi-phase fluids may include some combination of oil, water, and gas. In process industries, oil and gas industries and other such areas, it is often necessary to accurately measure fractions and flow rate of phases of the multi-phase fluid flowing inside a pipeline. With smaller and deeper oil/gas wells with higher water content becoming more common around the globe, there is an enhanced need for multi-phase flow measurement techniques.
Commercially available sensors for measuring fractions in fluids in the petroleum industry are based on a variety of principles (either a single technique or a combination of several techniques). For example, impedance sensors, capacitive and/or inductive sensors, dual-energy gamma sensors, venturi meters, and microwave sensors (attenuation/phase/resonance) have all been used. Currently, there are numerous microwave-based flow metering sensors available offering varying degrees of sensitivity, complexity and costs.
Accuracy of current fraction measurement systems may be affected by the presence of saline content in the water phase of the multi-phase fluid. Presence of saline content leads to changes in the permittivity of the multi-phase fluid. If salinity is not determined accurately, changes in permittivity can be incorrectly attributed to changes in water fraction for instance. The measurements therefore need to be compensated for salinity for accurate fraction measurements.
Further, salinity determination helps users of the measurement facility to take control actions. Control actions pertaining to descaling of conduits are particularly dependent on measuring salinity in the multi-phase fluids flowing through the conduits. Metallic conduits may experience corrosion due to deposition of saline material on the inner surface of the conduits that is exposed to the multi-phase fluid. Hence, measuring saline content in the fluid flowing through the conduit is important.
Current salinity measurement systems include systems that are dependent on using phase differences observed in electromagnetic waves received at different sensing antennas. However, phase difference methodologies have been observed to provide accurate results in limited cases for multi-phase fluids with low dielectric losses (for example: oil-continuous fluids and wet gas streams).
Other existing systems and methods include determining reflection coefficients of reflected electromagnetic waves received from the multi-phase fluid, determining conductivity from the reflection coefficients and estimating salinity based on a conductive loss term in the permittivity associated with the fluid. Permittivity of water (∈rw) can be expressed as follows:
            ɛ      r        ⁡          (      w      )        =            ɛ      r      ′        -          j      ⁡              (                              ɛ            r            ″                    +                      σ                          ω              ⁢                                                          ⁢                              ɛ                0                                                    )            
where, ∈′r corresponds to a real part of permittivity, ∈″r corresponds to the dielectric loss term,
  σ      ω    ⁢                  ⁢          ɛ      0      represents the conductive loss term, where σ is the conductivity, ω is the angular frequency and ∈0 is the permittivity of free space.
As can be seen from the equation the conductive loss term is inversely proportional to frequency and hence decreases with an increase in frequency and becomes an insignificant value at higher frequencies. On the other hand, the dielectric loss term increases with frequency until resonance is achieved and then decreases with frequency. The resonance frequency, depending on the composition of multi-phase fluids, may be in the range of tens of GHz. When the operating range are in the range where the dielectric loss term is significant, current methods that depend on determining only the conductive loss term and ignore the dielectric loss term may produce erroneous results.
Accordingly, there is an ongoing need for multi-phase flow metering systems and methods that determine saline content in multi-phase fluids across all frequency ranges and for multi-phase fluids that contain substantial amounts of lossy medium.